Various apparatus are known for use in boring and cutting holes through workpieces. Conventional boring apparatus typically include a rotating cutting head which moves along an axis while it rotates thereabout to contact a portion of a workpiece. A cutting blade extending forwardly from the cutting head parallel to and spaced from the axis performs the cutting operation, such that the rotation of the cutting head moves the blade along a circular path in a plane perpendicular to the axis. The hole is formed by advancing the cutting head along the axis towards the workpiece as it is rotated, until the cutting blade is moved completely through the workpiece.
A major disadvantage of conventional boring tools is that both the hole which is formed, and the waste slug or plug which is cut from the workpiece, have side edges parallel to the axis. As a result, the waste slug or plug often falls through the hole into the workpiece. This is particularly undesirable where the tool is used to form holes in blow moulded hollow workpieces as often the plugs are difficult to retrieve and must be manually removed.
In an attempt to prevent waste plugs from falling into the workpiece, some boring tools are provided with a slug retainer or piercing pin on the cutter head. The piercing pin typically includes an enlarged arrow-head like tip portion, and has a length selected so that it pierces the workpiece wall before the slug is fully cut out. Once the cutting operation is completed, the cutting head is moved away from the workpiece carrying the cut waste plug thereon. Boring tools which incorporate conventional plug retention mechanisms suffer the disadvantage in that they require time consuming recalibration for each different workpiece line, to ensure that the piercing pin moves the correct distance through the workpiece sidewall.
A further disadvantage with conventional boring tools exists in that they are not well suited for use with workpieces which are formed from blow moulded plastics, such as high density polyethylene or polypropylene, and which are used to make plastic automotive fuel tanks, brake-fluid reservoirs as well as other non-automotive parts. The blow moulding process inherently produces workpieces which differ in precise structure from one workpiece to the next. Because conventional boring apparatus move through a series of preset movements, their performance is dependent on the workpiece wall thickness, surface finish, plastic temperature and location of the portion of the workpiece wall which is to be cut. As the blow moulding process produces variations in wall thickness, surface thickness and even wall configuration, it is almost impossible to preset most boring apparatus to optimally perform cutting operation on every workpiece of a given product line. Depending on the variations in wall configuration and/or thickness, existing boring tools suffer the disadvantage that if the system is not correctly set up, cutting may commence without proper coupling between the piercing pin and plug. The waste plug may therefore be pushed into the workpiece and/or stripped from the slug retainer pin inside the workpiece when the cutting head is retracted.